There is increasing demand for purified water for various industries such as semiconductors, pharmaceuticals, and power generation. More frequently, these industries are located near centers of increasing population. These factors combine to put increased demands on available water supplies, and sophisticated water purification systems are needed to process feedwaters of declining quality (increasing salinity). Additionally, economic factors are demanding that water purification systems become less expensive to build and operate, and environmental factors are demanding that these systems utilize available feedwaters with greater efficiency and generate less waste. These demands can be met by providing improved water purification technologies and systems which economically process and recover a substantial fraction of feedwater as a purified product water, even in areas where the quality of the feedwater is declining.
The present invention provides for economical purification of feedwaters which contain significant concentrations (typically 30 ppm or greater) of silica by means of reverse osmosis (RO), and allows recovery of up to 90% or more of feedwaters as purified product without deposition of insoluble, amorphous silica within the reverse osmosis equipment.
Silica is ubiquitous in natural waters. The solubility limit of such silica in most waters is approximately 125 ppm. However, the chemistry of silica is complex; the actual solubility limit of silica in a particular water is variable and dependent upon numerous factors including temperature, pH, ionic composition, ionic strength, etc. When silica-containing waters are concentrated by means of conventional reverse osmosis and the relevant silica solubility limit is exceeded in the RO retentate, silica can precipitate and form “scale” on exposed surfaces of the system. RO system performance is then greatly degraded, and it is expensive and difficult to remove such scale once it has formed. For many natural waters with native silica concentrations of 30-80 ppm, the maximum practical recovery of purified water by conventional RO is limited to about 35-70%.
Former methods for achieving high recovery of silica-containing feedwaters as purified product by means of RO have relied on extensive and expensive pretreatment processing of feedwaters prior to RO, or alternatively on the addition of expensive “anti-scalant” chemicals. Such former preferred methods typically require adjusting the pH of the feedwaters to 10 or greater in order substantially to ionize silica and thereby maintain silica in solution. Levels of hardness in such feedwaters must first be reduced to very low concentrations, however, to prevent a different problem, namely deposition of mineral scale when the pH of the feedwaters is increased to relatively high pH levels. It is desirable also in such former methods to reduce the level of dissolved carbon dioxide in the feedwaters to reduce chemical usage needed for increasing the pH. Reference may be made to the work of Mavrov et al. (Desalination, 123 (1999) 33-43), and to U.S. Pat. No. 5,925,255 (Mukhopadhyay), which are incorporated herein by reference, for further discussions of such former methods.
The success of addition of “silica anti-scalant” chemicals to RO feedwaters is generally limited to applications producing RO retentates with not more than about 200-300 ppm silica. Thus high recovery (e.g., 90% or greater) of feedwaters containing significant concentrations of dissolved silica is generally not possible with silica anti-scalants. Reference may be made to Darton (Desalination, 124 (1999) 33-41), which is incorporated herein by reference.